1. Field of the Invention
The present invention relates to a technique capable of illuminating a large-area illumination surface with laser light that is high in uniformity. The invention is particularly suitable for annealing of a semiconductor film.
2. Description of the Related Art
In recent years, extensive studies have been made of techniques of crystallizing or improving the crystallinity of an amorphous semiconductor film or a crystalline semiconductor film (i.e., a semiconductor film that is not a single crystal but is crystalline, for example, polycrystalline or microcrystalline) by subjecting it to laser annealing. A silicon film is widely used as such a semiconductor film.
Glass substrates have an advantage that they are less expensive and higher in workability and enable formation of a large-area substrate more easily than quartz substrates that have widely been used conventionally. This is the reason for the above-mentioned studies. The reason why lasers are used for crystallization is because of low melting points of glass substrates. Lasers can crystallize a non-single-crystal film by applying high energy without changing the substrate temperature to a large extent.
Since crystalline silicon films formed by laser annealing have high mobility, they are widely used in monolithic liquid crystal electro-optical devices in which, for example, both of pixel driving TFTs (thin-film transistors) and driver circuit TFTs on a single glass substrate by using such a crystalline silicon film. Having a number of crystal grains, such a crystalline silicon film is called a polysilicon film or a polycrystalline semiconductor film.
On the other hand, because of high mass-productivity and advantages in industrial applicability, a laser annealing method is used by preference in which a laser beam emitted from an excimer laser or the like having large output power is processed by an optical system so as to form a several centimeter square spot or a line of several millimeters in width and tens of centimeters in length on an illumination surface and the illumination surface is scanned with the laser beam (i.e., the laser beam illumination position is moved relative to the illumination surface).
In particular, in contrast to the case of using a spot-like laser beam that requires two-dimensional scanning, the use of a linear laser beam allows the entire illumination surface to be illuminated by one-dimensional scanning in the direction perpendicular to the longitudinal direction of the linear laser beam, whereby high mass-productivity is obtained. The scanning in the direction perpendicular to the longitudinal direction is employed because it is most efficient. Because of the high mass-productivity, the use of a linear laser beam is now becoming the mainstream in the laser annealing technology.
Laser-annealing a non-single crystal semiconductor film by scanning it with pulse laser beams that have been processed into a linear shape have several problems.
For example, there is a problem that, in general, when a laser beam is applied to the surface of a semiconductor coating formed on a substrate that has differences in height due to undulation or the like of the substrate, the laser beam does not focus on the surface locally.
This problem causes a case that laser annealing is not performed uniformly over the entire film surface. For example, when a linear laser beam is used in laser annealing, there occurs a marked phenomenon that stripes are formed at overlapping portions of beams. The semiconductor characteristics of the film vary very much from one stripe to another.
This problem is particularly serious in a case where laser light is applied to large-area substrates, because differences in height are relatively large in large-area substrates. For example, a substrate of 600 mm×720 mm has undulation of about 100 μm, which is very large for a laser beam used having a certain kind of feature.
A specific state of a laser beam in the vicinity of the focal point will be described below. The energy profile, at and in the vicinity of the focal point, of a laser beam depends on the form of an optical system that produces the laser beam.
For example, in the case of a beam produced by simply converging a laser beam into a linear shape, a deviation from the focal point affects the beam width and the energy density. FIG. 1A shows an optical system that simply converge a laser beam into a linear shape. Reference numeral 100 denotes a laser beam, 101 and 102 denote cylindrical lenses for expanding the laser 100, and 103 denotes a cylindrical lens for converging it in the width direction.
In general, in this type of optical system, the energy uniformity on an illumination surface 104 is poor because the laser beam 100 is simply converged into a linear shape. When this type of optical system is used, it is required that the laser beam 100 before being processed into a linear shape be very high in energy uniformity. Since a deviation from the focal point varies the energy density on the illumination surface 104, it is not desirable to form a laser beam by an optical system having the above type of configuration.
FIG. 1B shows an optical system in which a concave cylindrical lens 105 is added to the optical system of FIG. 1A. In a case where a linear beam is formed in the manner shown in FIG. 1B, the concept “focal point of a laser beam” itself is meaningless because the laser beam is parallel in the vicinity of an illumination surface 106. Therefore, the problem of a deviation from the focal point does not occur either. However, the laser beam energy density is high at the lens 105 from which a linear laser beam is output, and the lens 105 is not so durable as to sustain such a high energy density. Therefore, at present, this type of optical system is not practical. Further, when this type of optical system is used, it is required that an original laser beam (i.e., a laser beam before being processed into a linear shape) be very high in energy uniformity.
In the above two examples, it is required that a laser beam before being processed into a linear shape be very high in energy uniformity. At present, no laser beam generating device is available that generates a laser beam having sufficiently high uniformity for the purpose of annealing a semiconductor film. The above configurations thus require development of a new technology.
Because of low uniformity in the energy profile of a linear laser beam, at present the above two examples are not suitable for annealing of a semiconductor film. Next, a description will be made of examples of optical systems that are currently in actual use.
An optical system having a configuration shown in FIG. 2A forms a linear laser beam. The configuration of this optical system is such that a laser beam is divided vertically and horizontally and divisional beams are combined into a single beam on an illumination surface while being processed into a linear shape individually. This configuration makes it possible to uniformize the energy profile of a linear laser beam.
FIGS. 3A-3C show energy profiles, in the width direction, of a linear laser beam formed by the lens group of FIG. 2A at the focal point (combined focal point) and positions slightly deviated from the focal point. In the cross-sections at the positions slightly deviated from the combined focal point, the energy profile assumes a step-like shape because divisional beams are not completely combined into a single beam.
FIGS. 3A-3C show energy profiles at a position immediately upstream of the focal point, at the focal point, and at a position immediately downstream of the focal point, respectively, and correspond to broken lines a-c in FIG. 2B, respectively. The term “focal point of a linear laser beam” as used here means a plane where divisional beams are substantially combined into a single beam. In many cases, the width of a linear laser beam is set at 1 mm or less, because it is generally required to have high energy density. Therefore, the beam shapes shown in FIGS. 3A and 3C are approximately congruent with each other.
When a laser beam having the energy profiles as shown in FIGS. 3A-3C is applied to a semiconductor film, the annealing effect at a position where a central portion, in the width direction, of the linear laser beam is applied is entirely different from that at a position where an end portion of the linear laser beam is applied. To perform laser annealing by using such a laser beam with as high a level of uniformity as possible, a measure is commonly taken that laser beams are applied to a semiconductor film while being superimposed on each other.
Good results are obtained by superimposing linear laser beams on each other so that a portion of a semiconductor film that has been illuminated with an end portion of one linear laser beam is again illuminated with a central portion, in the width direction, of another linear laser beam. Since a laser generating device used is an excimer laser which is a pulsed laser, the entire semiconductor film can be illuminated with laser light by superimposing linear laser beams on each other on the semiconductor film.
To anneal the entire semiconductor film by using linear laser beams of the above kind with a highest level of uniformity, it is important to superimpose illumination regions of respective linear laser beams while moving the semiconductor film at a pitch x that is 1/20 to ⅕ of the width W of the linear laser beams. That is, it is necessary to apply laser beams so as to satisfy a condition W/20≦x≦W/5.
Particularly good results are obtained by superimposing laser beams at a pitch x that is approximately 1/10 of the width W. However, even if laser illumination is performed under such a condition, there still occurs a marked phenomenon that stripes are formed at overlapping portions of beams.
FIG. 4 shows such stripes formed on a 5-inch-square substrate of 0.7 mm in thickness. The substrate had asperity (undulation) of about 20 μm. The stripes are seen depending on the manner of reflection when the surface of a laser-annealed silicon film is observed.
The stripes of FIG. 4 appeared when linear, XeCl excimer laser beams extending in the right-left direction in FIG. 4 were applied with scanning in the top-to-bottom direction in FIG. 4.
It is considered that the horizontal stripes of FIG. 4 result from the manner of overlapping of shots of pulse laser beams.
When an active matrix liquid crystal display was manufactured by forming thin-film transistors by using a silicon film having a stripe pattern as shown in FIG. 4, there occurred a problem that similar stripes appeared in a displayed image.
This problem is more serious in a large-area (600 mm×720 mm) substrate of 0.7 mm in thickness because the surface has differences in height of about 100 μm.
In general, in forming a linear laser beam, a laser beam having a rectangular cross-section is processed into a linear shape by causing it to pass through a proper optical system. A rectangular beam having an aspect ratio of 2 to 5 is shaped into a linear beam having an aspect ratio of 100 or more by an optical system shown in FIG. 2A, for example. The optical system of FIG. 2A is so designed as to also uniformize the beam energy profile.
The apparatus shown in FIG. 2A has a function of applying, to an illumination surface, a linear beam that has been obtained by shaping a laser beam (generally rectangular in this state) emitted by an oscillator 201 by an optical system having lenses 202-204, 206, and 208. Reference numerals 205 and 207 denote a slit and a mirror, respectively.
The cylindrical lens group (also called a multi-cylindrical lens) 202 has a function of dividing a beam into many beams. The many divisional beams are combined by the cylindrical lens 206 as the final lens.
This structure is required to improve the uniformity of the beam intensity profile. The combination of the cylindrical lens group 203 and the cylindrical lens 204 has basically the same function as the combination of the cylindrical lens group 202 and the cylindrical lens 206.
That is, while the combination of the cylindrical lens group 202 and the cylindrical lens 206 has a function of improving the uniformity of the intensity profile in the longitudinal direction of a linear laser beam, the combination of the cylindrical lens group 203 and the cylindrical lens 204 has a function of improving the uniformity of the intensity profile in the width direction of a linear laser beam.
An optical system having a role of uniformizing the beam energy profile is called a beam homogenizer. The above-described optical systems shown in FIG. 2A are beam homogenizers. The energy profile is uniformized by dividing an original rectangular beam, expanding divisional beams, and superimposing the expanded divisional beams one on another.
A linear laser beam formed by the above optical system has energy profiles as shown in FIGS. 3A-3C at the focal point and the positions slightly deviated therefrom. As seen from FIGS. 3A-3C, the energy profiles at the positions slightly deviated from the focal point are different from that at the focal point. These differences further the formation of a stripe pattern.
The configuration of the lenses shown in FIG. 2A is the basic one, and another optical system may be added thereto or part of the lenses may be replaced by other lenses having a similar action. Or the configuration of FIG. 2A may be used as part of the entire optical system. For example, the cylindrical lens group 202 and the cylindrical lens 203 which are convex lenses may be replaced by concave lenses or concave/convex-mixed lenses.
Where lenses that are not congruous with each other as typified by concave/convex-mixed lenses are used, the lenses should have such structures that beams obtained by the lenses by processing a parallel beam have the same divergent angle. Otherwise, when divisional beams are recombined, beams having different sizes and shapes are superimposed one on another, as a result of which the outline of a resulting beam becomes unclear.
A laser beam may be divided by other methods than using a cylindrical lens. For example, as shown in FIG. 10, the cylindrical lens group 203 and the cylindrical lens 204 shown in FIG. 2A may be replaced by a multi-phase prism 1001 having approximately the same action. Because of a decrease in the number of lenses, this optical system has several merits. For example, the loss of light quantity can be reduced and the alignment adjustment of the optical system can be made easier.
An object of the invention is to reduce the degree of unevenness in laser light illumination as shown in FIG. 4.
Another object of the invention is to provide an optical system which can prevent unevenness in laser light illumination by minimizing a variation in energy profile in the vicinity of the focal point of a laser beam (see FIGS. 3A-3C).
FIG. 2B shows an optical path of a laser beam from the cylindrical lens (hereinafter referred to as “final lens”) 208 that is disposed at the end of the optical system having the optical system of FIG. 2A as the basis to the illumination surface. FIG. 2B shows how a plurality of beams are combined into a single linear beam on the illumination surface. If the illumination surface is deviated from, that is, located upstream or downstream of, the combined focal point (i.e., the focal point of the entire optical system), that is, if the distance of the illumination surface from the final lens is varied, a plurality of laser beams are not completely combined into a single beam and hence a variation occurs in the energy profile. The cross-sections a-c shown in FIG. 2B correspond to the energy profiles of FIGS. 3A-3C, respectively.
Where a substrate of 600 mm×720 mm is used, in many cases the height difference r of a semiconductor film to be illuminated amounts to about 100 μm when the substrate is placed on a flat stage. In this case, it is necessary to design an optical system that prevents the crystallization state of a semiconductor film from reflecting a deviation from the focal point in the ranges having a length 100/2=500 μm that are immediately upstream of and downstream of the combined focal point of a laser beam.
For convenience in substrate transport, it is preferable that the stage to be mounted with a substrate be of a 3-point support type (depending on the rigidity of a substrate, there may occur a case that supporting at four or more points is even preferable). However, a substrate mounted on such a stage is warped and is thereby increased in undulation (the substrate has undulation originally). In such case, naturally it is necessary to perform laser annealing by using a laser beam that enables satisfactory crystallization even if undulation of such a level exists. When a substrate of 600 mm×720 mm in size and 0.7 mm in thickness was mounted on a 3-point support stage, the substrate surface had apparent height differences of about 1,000 μm.
FIGS. 5A and 5B show two final lenses having different focal lengths. It is seen from optical paths of laser beams that pass through the two final lenses that the variation in the energy profile of a laser beam in the vicinity of the illumination surface is smaller when the ratio of the distance F between the generatrix of the final lens and the semiconductor film to the size D, in the direction perpendicular to the generatrix of the final lens, of a region on the final lens on which the laser beam is incident is larger.
This point will be described below with reference to FIGS. 5A and 5B. In FIGS. 5A and 5B, symbol z represents a distance between two beam cross-sections that are located close to and on both sides of the combined focal point and in which the beam width is w* (the beam width is W at the focal point).
A relationship z1≦z2 holds when F1/D1≦F2/D2. That is, the variation in beam width with respect to the deviation from the combined focal point is smaller in the case of FIG. 5B.
At the positions deviated from the combined focal point by z/2, the beam width of a linear laser beam decreases by Δ=W−w* from that at the combined focal point. Since DW, an approximation Δ(z)=(z/2)×D/F can be made.
As shown in FIGS. 3A-3C, the bean width w* at a position deviated upstream or downstream from the combined focal point by z/2 is defined as a width of a region where the energy density is substantially the same as at the combined focal point. In this specification, parameter Δ is called a beam width variation amount.
Symbol Δ(r), rather than Δ, particularly means a beam width variation amount that occurs when a laser beam is applied to a semiconductor film having a height difference r.